1. Field of the Invention
The present invention relates to a method and apparatus for deriving pressure signals relative to the blood pressure within the left ventricle of a patient's heart and employing those signals to control pacing rate of a pacemaker in response to the physiologic needs of the cardiovascular system and/or to confirm the existence of a pathologic tachyarrhythmia and trigger the delivery of an appropriate therapy, such as anti-tachyarrhythmia pacing, cardioversion or defibrillation.
2. Description of the Prior Art
Advances in the treatments of bradyarrhythmias (slow heart beat) and tachyarrhythmias (fast heart beat) with implanted devices capable of detecting each condition and providing the appropriate therapy resulted in numerous advances in the art since simple fixed rate pacemaker were first implanted about thirty years ago. The control of the heart's rhythm by monitoring both electrical and mechanical heart function has been a goal of researchers in the field over that same period of time. For example, the 1961 pamphlet by Dr. Fred Zacouto, Paris, France, "Traitement D'Urgence des Differents Types de Syncopes Cardiaques du Syndrome de Morgangni-Adams-Stokes", (National Library of Medicine), describes an automatic pacemaker and defibrillator responsive to the presence or absence of the patient's blood pressure in conjunction with the rate of the patient's electrocardiogram. Very generally, a simple algorithm employing the patient's heart rate as evidenced by the electrical R-waves and the patient's blood pressure pulse was employed to: (1) operate a pacemaker pulse generator at a fixed rate in the presence of both signals recurring at less than a minimum rate or escape interval; (2) trigger the delivery of a defibrillation shock to the heart in the presence of a heart rate exceeding a tachyarrhythmia detect upper rate threshold in conjunction with the absence of a blood pressure signal over a variable period (such as thirty seconds); and, (3) inhibit both the pacemaker and defibrillator in the presence of both R-waves and blood pressure pulses recurring at a frequency exceeding the lower rate threshold but falling below the tachyarrhythmia detect threshold. It was long recognized earlier in medicine that the patient's blood pressure and electrocardiogram constituted the two most familiar and direct diagnostic tools for assessing the condition of the patient's cardiovascular system.
In this regard, it was also recognized early in the history of cardiac pacing that the patient dependent upon fixed rate pacing stimulation suffered cardiovascular insufficiency as his heart was able to increase its output (cardiac output) by only a limited amount in response to physiologic need. In normal hearts, the cardiovascular system responds to physiologic need by increasing both the heartbeat rate and its volume and systolic pressure (thereby increasing stroke volume and cardiac output) in response to physiologic need and as the heartbeat rate is limited in the pacing dependent patient to the fixed pacer rate, the heart's blood pressure and volume could increase proportional to physiologic need only to a limited extent. Thus, it was suggested by Juhasz in his 1965 article "Development of Implanted Cardiac Pacemakers", Digest of 6th Int'l Conf. on Medical Electrics and Biological Engineering, 1965, Tokyo, pp. 85-86, that blood pressure, among other parameters of the cardiovascular system, could be used as a forward transfer control value to vary pacing rates as a function of the blood pressure value, thus releasing the heart from the constraint imposed by the fixed base or lower pacing rate and allowing it to beat up to the pulse generator's upper pacing rate limit.
These early researchers were followed by numerous examples of the use of pressure signals of one form or another to control pacing rate or verify the presence of a tachyarrhythmia and trigger the delivery of appropriate therapies. For example, it has been proposed to sense pressure in the right atrium and to utilize a signal derived therefrom to affect and control right ventricular pacing as disclosed in Cohen U.S. Pat. No. 3,358,690. In addition, the Zacouto U.S. Pat. No. 3,857,399 (FIG. 19) discloses a pressure sensor on an extension of a pacing lead adapted to be forced into or through the ventricular septum to measure the intramyocardial pressure within the septum, and/or the actual left ventricular pressure. The signal derived from one or both of these sensors represents an average or mean pressure that varies over relatively long periods of time in a manner similar to that described in the Kresh PCT Publication No. W087/01947. More recently, the publication of Todd J. Cohen, entitled "A Theoretical Right Atrial Pressure Feedback Heart Rate Control System to Restore Physiologic Control to a Rate Limited Heart", PACE, Vol. 7, pp. 671- 677, July-August, 1984, discloses a system for comparing the mean right atrial pressure signal with a baseline signal and developing an error signal which, after processing, is used to control the pacing rate.
In addition, the microprocessor based implantable pacemaker and ventricular pressure sensing lead disclosed in Koning et al, U.S. Pat. No. 4,566,456, relates right ventricular systolic pressure, the gross rate of change over time of the pressure (.DELTA.P/.DELTA.t) and/or the time derivative (dP/dt) of the systolic pressure with the rate needed to produce the desired cardiac output. Koning, in one algorithm, detects the right ventricular systolic pressure peak valves, averages N peak values and compares the current average to the preceding stored average value to detect the change in average pressure over time (.DELTA.P/.DELTA.t). That signal is employed to "look up" a .DELTA.R or pacing rate change used to modify the pacing rate R.
More recently, Cohen U.S. Pat. No. 4,899,751 discloses a pacing system relying on a pressure signal from a pressure sensor located in the cardiovascular system, including the four chambers of the heart, coupled with signal processing circuitry for developing short term and long term mean (or average) pressure related control signals therefrom. The escape interval or rate of the pacemaker is controlled as a function of the difference between the short term and long term mean pressure values. Cohen, U.S. Pat. No. 4,899,752, provides a somewhat different algorithm in that the current mean pressure values are compared against fixed threshold values and the difference is employed to modify the pacing rate.
Medtronic U.S. Pat. Nos. 4,407,296, 4,432,372 and 4,485,813 describe various transvenous pressure sensors with associated pacing electrodes adapted to be positioned in a heart chamber to develop pressure values to control the operation of rate responsive pacemakers or to detect pathologic tachyarrhythmias and trigger the delivery of appropriate therapies.
In regard to the use of a blood pressure related signal detected within a heart chamber to confirm the detection of a tachyarrhythmia and trigger the delivery of an appropriate therapy, the initial system proposed by Mirowski et al in U.S. Pat. No. Re 27,757 relied upon the decrease in the amplitude of a pulsatile (systolic) right ventricular pressure signal below a threshold over a predetermined period of time (.DELTA.P/.DELTA.t) to commence the charging of a high energy output capacitor and deliver a shock to the heart if the pressure signal did not increase above the threshold during the charging time. The short lived pressure sensor available to Mirowski at that time was abandoned in favor of electrocardiogram rate and morphology detection.
More recently, the use of intramyocardial pressure and left ventricular pressure has been explored by a research group from Belgium (see, for example, the paper by Denys et al entitled "Ventricular Defibrillation Detection by Intramyocardial Pressure Gradients" in PROCEEDINGS OF THE SEVENTH WORLD SYMPOSIUM ON CARDIAC PACING, pp. 821-826, Verlag, 1983, and subsequent papers, such as "Automatic Defibrillator, Antitachy Pacemaker and Cardioverter", COMPUTERS AND CARDIOLOGY, IEEE COMPUTER SOCIETY PRESS, pp. 45-48, Oct. 7-10, 1986, and other papers by this group. This group has advocated the use of left ventricular impedance or pressure or a left ventricular pressure related signal over right ventricular pressure, and they resorted to use of ventricular intramyocardial pressure because of the difficulty of directly measuring pressure in the left ventricle and atrium.
In addition, a Japanese group has published papers such as "Design for an Implantable Defibrillator Using a Novel Heartbeat Sensor", Japanese Journal of Medical and Biological Engineering, 1984, pp. 43-48, by Makino et al. The Japanese group's sensor detects the pressure in the right ventricle using a catheter born electrode, or microphone, heartbeat sensor. The absence of a heartbeat for 3.5 seconds causes the fibrillation detector to switch the high voltage converter into operation.
The comparison of a current average pressure value to a longer term average control value derived from the heart during normal sinus rhythm to detect ventricular arrhythmias and trigger cardioversion/defibrillation therapies in response to a significant decrease in the current value was proposed by Olson et al, in "Automatic Detection of Ventricular Fibrillation with Chronic Pressure Sensors", (abstract), JACC, Vol. 7, No. 2, Feb., 1986, p. 182A.
More recently Cohen, U.S. Pat. No. 4,774,950, describes a system employing mean pressure values from any of the four chambers of the heart representative of the long-term mean base line pressure and the short-term current mean pressure to indicate or confirm the indication of a tachyrhythmia and to trigger cardioversion/defibrillation shock therapies when the difference between the two mean pressure values exceeds a predetermined threshold value.
The truest indication of the degree of hemodynamic compromise of the malfunctioning heart is the left ventricular pressure which is measurable only with some difficulty. For example, Zacouto, Kresh, the Belgian group and Cohen (in the '751 and '950 patents) all have sought in one way or another to determine the left ventricular pressure by locating a pressure sensor within the left ventricle or within the myocardial tissue. Placing and retaining a pressure sensor in either location involves some risk that the high pressure, left ventricular chamber will be breached at the point of penetration causing the patient to hemorrhage as expressly commented on by the Belgian group. Thus with current technology, it is undesirable to so situate a pressure sensing transducer. However, the desirability of measuring is directly as possible the left atrial o ventricular blood pressure remains high.